In recent years, with an aim toward effective utilization of energy for greater environmental conservation and reduced usage of resources, a great deal of attention is being directed to electric power smoothing systems for wind power generation or overnight charging electric power storage systems, household dispersed power storage systems based on photovoltaic power generation technology, and power storage systems for electric vehicles.
The number one requirement for cells used in such power storage systems is high energy density. The development of lithium ion batteries is advancing at a rapid pace, as an effective strategy for cells with high energy density that can meet this requirement.
The second requirement is a high output characteristic. For example, in a combination of a high efficiency engine and a power storage system (such as in a hybrid electric vehicle), or a combination of a fuel cell and a power storage system (such as in a fuel cell electric vehicle), a high output discharge characteristic is required for the power storage system during acceleration.
Electrical double layer capacitors are currently under development as high output power storage devices.
Electrical double layer capacitors that employ active carbon in the electrodes have output characteristics of about 0.5 to 1 kW/L. Such electrical double layer capacitors have high durability (especially cycle characteristics and high-temperature storage characteristics), and have been considered optimal devices for fields requiring the high output mentioned above. However, their energy densities are no more than about 1 to 5 Wh/L. Even higher energy density is therefore necessary.
On the other hand, nickel hydrogen cells employed in current hybrid electric vehicles exhibit high output equivalent to electrical double layer capacitors, and have energy densities of about 160 Wh/L. Still, research is being actively pursued toward further increasing their energy density and output, and increasing their durability (especially stability at high temperatures).
Research is also advancing toward increased outputs for lithium ion batteries as well. For example, lithium ion batteries are being developed that yield high output exceeding 3 kW/L at 50% depth of discharge (a value representing the state of the percentage of discharge of the discharge capacity of a power storage element). However, the energy density is 100 Wh/L or less, and the design is such that high energy density, as the major feature of a lithium ion battery, is reduced. The durability (especially cycle characteristic and high-temperature storage characteristic) is inferior to that of an electrical double layer capacitor. In order to provide practical durability, therefore, they can only be used with a depth of discharge in a narrower range than 0 to 100%. Because the usable capacity is even lower, research is actively being pursued toward further increasing durability.
There is strong demand for implementation of power storage elements exhibiting high energy density, high output characteristics and durability, as mentioned above, but the aforementioned existing power storage elements have their advantages and disadvantages. New power storage elements are therefore desired that can meet these technical requirements. Power storage elements known as lithium ion capacitors are being actively developed in recent years as promising candidates.
The energy of a capacitor is represented as ½·C·V2 (where C is electrostatic capacity and V is voltage). A lithium ion capacitor is a type of power storage element that uses a non-aqueous electrolyte containing a lithium salt (non-aqueous lithium-type power storage element).
It is a power storage element that carries out charge-discharge by:
non-Faraday reaction by adsorption/desorption of anion similar to an electrical double layer capacitor at about 3 V or greater, at the positive electrode, and
Faraday reaction by occlusion/release of lithium ion similar to a lithium ion battery, at the negative electrode.
An electrical double layer capacitor in which charge-discharge is accomplished by non-Faraday reaction at both the positive electrode and negative electrode has excellent input/output characteristics (it can perform charge-discharge of high current in a short period of time), but has low energy density. In contrast, a secondary battery in which charge-discharge is accomplished by Faraday reaction at both the positive electrode and negative electrode has excellent energy density but poor input/output characteristics. A lithium ion capacitor is a power storage element that aims to achieve both excellent input/output characteristics and high energy density by accomplishing charge-discharge by non-Faraday reaction at the positive electrode and Faraday reaction at the negative electrode.
The following, for example, have been proposed as lithium ion capacitors.
PTL 1 proposes a power storage element employing active carbon as the positive electrode active material, and as the negative electrode active material, a carbonaceous material obtained by occluding lithium by a chemical process or electrochemical process in a carbon material capable of occluding and withdrawing lithium in an ionized state. In that patent document, the carbon materials mentioned are natural graphite, artificial graphite, graphitized mesophase carbon microspheres, graphitized mesophase carbon fibers, graphite whiskers, graphitized carbon fibers, thermal decomposition products of furfuryl alcohol resin or novolac resin, and thermal decomposition products of polycyclic hydrocarbon condensation polymer compounds such as pitch coke. The procedure of occluding lithium in the carbon material beforehand will hereunder be referred to as “pre-doping”. This term will distinguish the procedure from “doping” in which lithium ion is occluded into the negative electrode during charge, and “undoping” in which it is released during discharge.
PTLs 2 to 6 each propose electrodes and power storage element using active carbon as the positive electrode active material, and using as the negative electrode active material a composite porous material with a carbonaceous material covering the surface of active carbon, where the negative electrode active material has been pre-doped with lithium.
The lithium ion capacitors using these negative electrode active materials have low internal resistance compared to lithium ion capacitors using other materials such as graphite for the negative electrode active material, and therefore high output characteristics are obtained.
PTL 7 discloses, as a negative electrode active material for a lithium ion secondary battery that can exhibit high output characteristics and high energy density, a composite carbon material comprising carbon black and a carbonaceous material. In the Examples in PTLs 8 and 9 it is explained that such composite carbon materials have specific weight-average meso/macropore specific surface areas and can exhibit high input/output characteristics when the composite carbon materials are used as negative electrode active materials for lithium ion capacitors.
Lithium ion capacitors with even more improved output characteristics and increased energy density are desired. One method for increasing the energy density is to lower the thickness of the negative electrode active material layer to reduce the cell volume, while maintaining the same energy. As the thickness of the negative electrode active material layer is lowered, however, the mass of the negative electrode active material per unit area of the negative electrode is reduced, and it is therefore difficult to maintain the same energy while also improving the output characteristic.